1. Field of the Invention
The present invention relates to an electric double layer capacitor in which electric double layers are formed at interfaces between electrodes and an electrolyte and electricity is stored in the electric double layers.
2. Description of the Related Art
An electric double layer capacitor is an electricity charging and discharging element. An example of such an electric double layer capacitor is disclosed in for example, Japanese Patent Laid-Open Publication No. HEI-10-294102 entitled xe2x80x9cElectricity Storing Elementxe2x80x9d.
The disclosed electric double layer capacitor comprises an electrode rolled body formed by overlapping a positive electrode plate, a negative electrode plate and a separator interposed therebetween to wind the same in a rolled manner. A lower end of the negative electrode plate is connected electrically to a negative collecting plate, which serves as a negative electrode terminal. An upper end of the positive electrode plate is connected electrically to a positive collecting plate, which serves as a positive electrode terminal. After the electrode rolled body in such a state is contained in a bottomed, cylindrical-shaped outer packaging can and an electrolyte is poured into the outer packaging can, the outer packaging can is closed by a cover. The positive collecting plate is electrically connected to the cover. The negative collecting plate is electrically connected to the outer packaging can.
With the above-mentioned electric double layer capacitor, heat generated due to electric resistance generated from the electrode rolled body and the like at the time of, for example, charging heats the electrolyte. Accordingly, to maintain the service life of an electric double layer capacitor over a long term, it is necessary to discharge the generated heat to the atmosphere. FIG. 15 hereof shows a schematic constitution, in which heat of the above-mentioned electric double layer capacitor is discharged.
In FIG. 15, an electric double layer capacitor 500 is constructed such that a negative collecting plate 505 is interposed between a negative electrode plate 502 of an electrode rolled body 501 and a bottom 504 of an outer packaging can 503 to electrically connect the negative electrode plate 502 and the negative collecting plate 505 to each other to connect the negative collecting plate 505 electrically to the bottom 504 of the outer packaging can 503.
The electric double layer capacitor 500 comprises a first connection 506, which electrically connects the negative electrode plate 502 and the negative collecting plate 505 to each other, and a second connection 507, which electrically connects the negative collecting plate 505 and the bottom 504 of the outer packaging can 503 to each other. Therefore, heat generated in the electric double layer capacitor 500 is discharged to the atmosphere from the outer packaging can through the first connection 506 and the second connection 507.
However, because the first connection 506 and the second connection 507 are small in area, heat quantity transferred through the first connection 506 and the second connection 507 becomes small. Therefore, heat transfer quantity generated in the electric double layer capacitor 500 is held down by the first connection 506 and the second connection 507, which is responsible for temperature rise in the electric double layer capacitor 500 to affect the service life thereof.
Further, with the electric double layer capacitor disclosed in Japanese Patent Laid-Open Publication No. HEI-10-294102, the positive collecting plate covers an upper end of the positive electrode plate, and so it acts as an obstacle when an electrolyte is filled into the electrode rolled body. Therefore, it takes time to fill the electrolyte into the electrode rolled body.
Japanese Patent Laid-Open Publication No. HEI-10-294102 further discloses a positive collecting plate having slits formed on both sides of radially extending convex ridges. Such a positive collecting plate will be described with reference to FIG. 16 hereof.
FIG. 16 shows, in an enlarged scale, a part of the positive collecting plate having the slits formed on both sides of the convex ridges. The convex ridges 601 of the positive collecting plate 600 are used to bend an upper end 604 of a positive electrode plate 603 in an electrode rolled body 602, and the convex ridges 601 are welded to bent portions 605 of the upper end 604.
Because the positive collecting plate 600 has slits 606, 606 on both sides of the convex ridges 601, it is possible to fill an electrolyte into the electrode rolled body 602 through the slits 606, 606.
Since both ends 605a of the bent portions 605 of the positive electrode plate 603 extend to the slits 606, 606, however, a part of the slits 606, 606 is closed by the both ends 605aof the bent portions 605. Therefore, when an electrolyte is filled into the electrode rolled body 602 from the slits 606, 606, the both ends 605aof the bent portions 605 act as an obstacle, and so it is difficult to efficiently fill the electrolyte from the slits 606, 606.
Further, with the electric double layer capacitor disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. HEI-10-294102, a leaf-spring shaped pressing member is mounted on a side of the cover in order to electrically connect the cover to the positive collecting plate, and the pressing member is brought into electrical contact with a central projection of the positive collecting plate. However, the pressing member contacts locally with the projection, and so the contact area therebetween is small. Therefore, electric current flowing in local contact portions between the pressing member and the projection is restricted.
To flow large electric current at contact portions is important in improving the performance of an electric double layer capacitor.
Hereupon, FIG. 17 shows a conventional electric double layer capacitor capable of flowing a relatively large electric current.
Referring to FIG. 17, an electric double layer capacitor 700 is constructed such that an outer packaging can 707 contains therein an electrode rolled body 701 in a state, in which a negative collecting plate 704 is electrically connected to a lower end of a negative electrode plate 702 of the electrode rolled body 701 and a positive collecting plate 705 is electrically connected to an upper end of a positive electrode plate 703. The negative collecting plate 704 is connected electrically to a bottom 708 of an outer packaging can 707. The positive collecting plate 705 is electrically connected to a cover 709. The outer packaging can 707 is filled with an electrolyte.
The positive collecting plate 705 is formed centrally with an upwardly extending projection 706. The projection 706 is inserted into a hole 709a formed in the cover 709. The projection 706 is welded to the cover 709. The projection 706 is formed to have a relatively large diameter to be large in cross sectional area. Therefore, a relatively large electric current can be made to flow to the projection 706 to improve the performance of the electric double layer capacitor 700.
It is necessary to reduce the electrode rolled body 701 in manufacturing error because it is required that a reference dimension Hi from the bottom 708 of the outer packaging can 707, which serves as a negative electrode, to a tip end 706a of the projection 706, which serves as a positive electrode, to be uniform. Therefore, a height h1 of the electrode rolled body must be kept uniform by decreasing the electrode rolled body 701 in manufacturing error.
However, the electrode rolled body 701 shown in FIG. 17 is constructed by overlapping and winding the positive electrode plate, negative electrode plate and a separator in a rolled manner and so displacement upon winding to generate manufacturing error is liable to occur. Therefore, to reduce the electrode rolled body 701 in manufacturing error to keep the height h1 of the electrode rolled body 701 uniform, an installation of high accuracy is necessary and it is difficult to hold down cost of an electric double layer capacitor due to the increased installation cost.
It is a first object of the present invention to provide an electric double layer capacitor capable of efficiently discharging heat generated in an interior thereof.
It is a second object of the invention to provide an electric double layer capacitor enabling efficiently filling an electrolyte into an interior of an electrode rolled body of the electric double layer capacitor.
It is a third object of the invention to hold down cost of an electric double layer capacitor.
In a first aspect of the present invention, there is provided an electric double layer capacitor having an outer packaging can containing therein an electrode rolled body obtained by interposing a separator between a pair of a positive electrode plate having a positive electrode foil, on both sides of which an active substance is applied, and a negative electrode plate having a negative electrode foil, on both sides of which the active substance is applied, and winding them, characterized in that the negative electrode foil is electrically connected directly to a bottom of the outer packaging can, and the negative electrode foil has a greater thickness than that of the positive electrode foil, which is not electrically connected to the outer packaging can.
In this manner, in the first aspect of the invention, the negative electrode foil is electrically connected directly to the bottom of the outer packaging can, so that locations of electric connection in a heat conductive path can be reduced to thereby provide an increase in heat transfer quantity. Accordingly, heat generated in the electric double layer capacitor can be efficiently transferred to the bottom of the outer packaging can from the negative electrode foil, and so heat generated in the electric double layer capacitor is efficiently discharged to the atmosphere. And the electric double layer capacitor is extended in service life.
Further, in the invention, the negative electrode foil connected to the bottom of the outer packaging can has a greater thickness than that of the positive electrode foil. Therefore, the negative electrode foil can be increased in cross sectional area to increase heat transfer quantity, so that heat in the electric double layer capacitor can be efficiently transferred to the bottom of the outer packaging can. Besides, the negative electrode foil is increased in thickness to be enhanced in rigidity, so that the electric double layer capacitor is improved in vibration-proof quality. Further, only the negative electrode foil is made thick and the positive electrode foil is made thin, whereby the electrode rolled body is not made large, and so the electric double layer capacitor does not become large-sized.
In a second aspect of the present invention, there is provided an electric double layer capacitor having an electrode rolled body obtained by overlapping and winding a pair of electrode plates, which serve as positive and negative electrodes, and positive and negative collecting plates, which are mounted to edges of the electrode rolled body and through which charging is effected on the electrode plates and discharging is effected from the electrode plates, characterized in that at least the positive collecting plate of the collecting plates includes a plurality of convex ridges extending radially from central portions thereof and projecting toward the electrode rolled body, and openings or notches formed to be positioned between adjacent convex ridges, and that the convex ridges are pressed against the electrode rolled body to form bent portions on upper and lower ends of the electrode plates, and the convex ridges are joined to the bent portions by welding to permit an electrolyte to be filled into the electrode rolled body through the openings or notches.
In this manner, in the second aspect of the invention, a plurality of the convex ridges are formed on the collecting plates, and the openings or notches are formed to be positioned between adjacent convex ridges. Accordingly, the convex ridges are pressed to form the bent portions on the upper and lower ends of the electrode plates, and the convex ridges are joined to the bent portions by welding to thereby enabling separating the openings or notches from the bent portions. Therefore, when an electrolyte is filled into the electrode rolled body from the openings or notches, the bent portions do not interfere with filling of the electrolyte and the electrolyte is rapidly filled.
In a third aspect of the present invention, there is provided an electric double layer capacitor comprising: an electrode rolled body formed by overlapping one of electrode plates on the other of electrode plates and winding the same in a rolled manner; a bottomed, cylindrical-shaped outer packaging can containing therein the electrode rolled body and connected electrically to one end of the one of electrode plates; a collecting plate connected electrically to the other of electrode plates of the electrode rolled body contained in the outer packaging can; and a cover covering the outer packaging can; and wherein the cover is formed centrally thereof with a hole, from which a cylindrical portion is extended outside and a projection formed centrally of the collecting plate is inserted into the cylindrical portion to an extent not to project from the cylindrical portion, and wherein the cylindrical portion and the projection are welded and sealed at a weld on an inner peripheral surface of the cylindrical portion.
In this manner, in the third aspect of the invention, the cylindrical portion is mounted to the cover, and the projection is formed on the collecting plate to be capable of being inserted into the cylindrical portion. Therefore, even when a relatively large manufacturing error is produced on the electrode rolled body, the manufacturing error of the electrode rolled body can be accommodated by moving the projection in that range, in which the projection will not project from the cylindrical portion, so that it is possible with a simple constitution to keep a reference dimension of the electric double layer capacitor uniform. Accordingly, when the electrode rolled body is wound, manufacturing error is allowable to some extent, and so it is possible to hold down cost of an electric double layer capacitor.